The purpose of this paper is to analyse the different phosphate compounds in sediments of freshwater systems and to quantify the processes by which they are formed and which may lead to the equilibrium between uptake and release. For the analysis a sequential extraction scheme is therefore developed, using chelators (such as NTA and EDTA) to extract first the 2 inorganic phosphates, i.e. the iron and the calcium bound phosphate. Fe(OOH) ≈ P is extracted with a Ca-NTA + dithionite solution, (resp. 0.02 M 1-1 and 0.05 M 1-1) and CaCO3 ≈ P with Na-EDTA (0.05 M 1-1). Both extractants are adjusted to the pH of the sediment to be analysed. Following these extractions, two pools of org-P can be identified, using subsequent extractions with H2SO4 (0.5 M 1-1 during 30 min) followed finally by one with NaOH (2 M 1-1, at 90° C during 5 min). The advantage of this scheme lies in the fact that the inorganic phosphates are extracted before the organic phosphates, together with the Fe(OOH) and CaCO3, so that no interactions between these adsorbents and phosphate can disturb the analysis of the organic phosphates after hydrolysis. The adsorption of ortho-phosphate onto freshwater sediments was studied. The influence of Fe(OOH) on this adsorption process was confirmed in the laboratory. It was found that this adsorption could be described satisfactory by the Freundlich adsorption isotherm : Pads where = A.(o-P)B Pads = o-P adsorbed onto the sediments or onto Fe(OOH) and A and B are constants We have quantified « A » and « B » by least squares fitting ; they are both functions of pH, although for B only moderately. If, however, B is set at 0.33 little precision is lost, and A can be approximated by : A = 23626* 10(-0.42 PH) . In waters with a low Ca2+ concentration, the adsorption onto the sediments can therefore now be quantified as a function of the annual loading, if the Fe(OOH) concentration in the sediments and the pH of the water are known. As Fe(OOH) plays such an important role in the adsorption process, the conversion, e.g. into FeS as usually occurs in anoxic sediments of shallow water bodies, will influence the adsorption strongly. The constant « A » does not only depend on the pH, but on the Ca2+ concentration as well. As yet no formula is available to quantify this influence. In hard water, the solubility of o-P will also be limited by the solubility product of apatite, Ca5(PO4)3OH. Using published data from the two hard water rivers Rhine and Rhone, we have found an 'apparent' solubility product of 10-50 , not taking into account the influence of the activity coefficient due to ionic strength. With this solubility product the maximal o-P concentration can be calculated as a function of the Ca2+ concentration in the water and the pH. Together with the equilibrium constants of the Fe(OOH) ≈ P adsorption complex, we have also calculated the maximal o-P concentration as it depends on the Fe(OOH) concentration in the sediments and the pH of the system. Combining these two functions, we have calculated a solubility diagram of the maximal o-P concentration in water in equilibrium with the Ca2+ concentration in the water, the Fe(OOH) concentration in the sediments and the pH. The solubility of dissolved ortho-phosphate appears to depend only on the Fe(OOH) concentration in the sediments, the Ca2+ concentration in the water (not the CaCO3 concentration in the sediments) and the pH. In 11 dutch lakes it appeared that the sum of (Fe(OOH) ≈ P + CaCO3 ≈ P) is available for phytoplankton growth. Furthermore we have demonstrated the existence of two organic phosphate pools in the sediments, the first acid soluble (ASOP) and the second alkaline soluble (ROP). We think that the first pool is a humic acid-phosphate complex although we can only present circumstantial experimental evidence. During the extraction, the phosphate of this pool is hydrolysed and goes into solution as o-P, which makes further identification difficult. The concentration of this pool in the sediments varied between 60 and 225 µg.g-1. During desiccation of the sediments of a marsh in the Camargue (Les Garcines), this pool disappeared and became inorganic phosphate. By using an enzymatic hydrolysis with phytase we could demonstrate that most of the second pool (ROP) was phytate, i.e. inositol hexaphosphate. Its concentration varied between 100 and 300 µg.g-1. Experimental evidence suggests that this phosphate is probably complexed with Fe(OOH). The existence of this complex can explain why this organic phosphate is not available for bacterial mineralisation, is therefore not biodegradable and can consequently accumulate in sediments. In shallow lakes and marshes receiving an important P loading, we can now quantify with the solubility diagram what will happen with the phosphate concentrations in water and sediments. It appears that during a first phase, nearly all phosphate will enter the sediments, while the concentration in the water increases only very slowly. Furthermore we can predict what will happen later'when the P-loading is stopped : depending on the water retention time, the situation will only improve very slowly and it may take even longer than the period of loading to return to the low, initial concentrations. In marshes without water renewal, the concentration will remain constant. The available phosphate may, however, still decline because of an accumulation of non-bioavailable phosphate, e.g. phytate.

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